The invention is based on a measuring element for a device for determining the mass or flow rate of a flowing medium, in particular the aspirated air of internal combustion engines.
In such devices, also known as hot-film air flow rate meters, the resistor tracks of a temperature sensor and a compensation resistor, on the one hand, and the resistor track of the measuring resistor on the other, together with two calibration resistors, form a Wheatstone bridge, whose bridge diagonal voltage is applied to a control amplifier. The output voltage of the control amplifier serves as the heating voltage for a heating resistor of the measuring element.
In a known measuring element for the above device, of the type referred to at the outset (German Patent Disclosure Document 36 38 138 A1), the distribution of the resistor tracks on the substrate is such that the resistor tracks are aligned parallel to one another and in succession in the flow direction. The resistor track for the compensation resistor is disposed between the resistor track for the temperature and the resistor track for the measuring resistor, on the same side of the substrate, and the resistor track for the heating resistor is disposed on the other side of the substrate, immediately opposite the resistor track for the measuring resistor. The various resistor tracks are separated from one another by means of slits in the substrate that extend crosswise to the flow direction; this provides temperature decoupling among the various resistor tracks. As a result of these separating slits, the substrate has three prongs of equal length, of which the first two prongs in the flow direction each have one resistor track, and the last prong in the flow direction has the resistor track for the measuring resistor and the heating resistor.
It has now been found that the characteristic of a measuring element of this kind has an unfavorable course in some regions, with a curvature that does not vary uniformly with an increasing flow rate of the medium. Regions of constant inclination alternate with those of variable inclination.
This characteristic is due to the trailing face located downstream, directly after the measuring resistor, at which face the flow is disturbed. A detachment region in which unsteady flow conditions prevail forms downstream of the trailing face, and this region is characterized by a stable vortex street, known as the Karman vortex street. A flow created by the negative pressure prevailing in the vortices and oscillating transversely to the main flow, leads to the aforementioned adulteration of the characteristic.